The present invention relates to the electrolytic production of metals such as aluminum. More particularly, the invention relates to electrolysis in a cell having an inert anode comprising oxides of nickel, iron and cobalt.
The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable and dimensionally stable anodes. Replacement of traditional carbon anodes with inert anodes allows a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also possible because inert anodes produce essentially no CO2 or CF4 emissions. Some examples of inert anode compositions are provided in U.S. Pat. Nos. 4,374,050, 4,374,761, 4,399,008, 4,455,211, 4,582,585, 4,584,172, 4,620,905, 5,279,715, 5,794,112 and 5,865,980, assigned to the assignee of the present application. These patents are incorporated herein by reference.
A significant challenge to the commercialization of inert anode technology is the anode material. Researchers have been searching for suitable inert anode materials since the early years of the Hall-Heroult process. The anode material must satisfy a number of very difficult conditions. For example, the material must not react with or dissolve to any significant extent in the cryolite electrolyte. It must not react with oxygen or corrode in an oxygen-containing atmosphere. It should be thermally stable at temperatures of about 1,000xc2x0 C. It must be relatively inexpensive and should have good mechanical strength. It must have high electrical conductivity at the smelting cell operating temperatures, e.g., about 900-1,000xc2x0 C., so that the voltage drop at the anode is low.
In addition to the above-noted criteria, aluminum produced with the inert anodes should not be contaminated with constituents of the anode material to any appreciable extent. Although the use of inert anodes in aluminum electrolytic reduction cells has been proposed in the past, the use of such inert anodes has not been put into commercial practice. One reason for this lack of implementation has been the long-standing inability to produce aluminum of commercial grade purity with inert anodes. For example, impurity levels of Fe, Cu and/or Ni have been found to be unacceptably high in aluminum produced with known inert anode materials.
The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.
The present invention provides an inert electrode including at least one ceramic phase material, which comprises oxides of nickel, iron and cobalt. The inert anode may also comprise at least one metal phase including copper and/or at least one noble metal.
An aspect of the invention is to provide an inert anode composition suitable for use in a molten salt bath. The composition comprises nickel, iron and cobalt oxide of the formula NixFe2yCozO(3y+x+z)xc2x1xcex4, where x is the molar amount of Ni, y is the molar amount of Fe, z is the molar amount of Co, and xcex4 is a variable which depends upon firing conditions.
Another aspect of the invention is to provide a method of making an inert anode composition. The method includes the steps of mixing nickel oxide, iron oxide and cobalt oxide, or precursors of such oxides, followed by calcining the mixture to form a ceramic material of the formula NixFe2yCOzO(3y+x+z)xc2x1xcex4, where x is the molar amount of Ni, y is the molar amount of Fe, z is the molar amount of Co, and xcex4 is a variable which depends upon firing conditions.
Some other aspects of the invention are to provide an electrolytic cell and an electrolytic process for producing commercial purity aluminum, utilizing the present inert anode materials.
Additional aspects and advantages of the invention will occur to persons skilled in the art from the following detailed description.